The standards for wireless local area network (WLAN) technologies are established by the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards committee. IEEE 802.11a and IEEE 802.11b respectively provide data rates of 54 Mbps and 11 Mbps using unlicensed bands of 2.4 GHz and 5 GHz. IEEE 802.11g adopts Orthogonal Frequency Divisional Multiplexing (OFDM) at 2.4 GHz and provides a data rate of 54 Mbps. IEEE 802.11n adopts Multiple Input Multiple Output (MIMO)-OFDM and provides a data rate of 300 Mbps for four spatial streams. IEEE 802.11n supports a channel bandwidth up to 40 MHz and provides a data rate of 600 Mbps.
A communication scheme for allowing operations of unlicensed users in frequency bands basically defined for use of licensed users is under discussion. Here, frequency bands temporarily not used by licensed users may be called white space and, particularly, white space in TV bands may be called TV white space (TVWS).
The IEEE 802/11af standard for defining operations of unlicensed devices in TVWS bands is currently under development.
TVWS includes very high frequency (VHF) bands (54 to 60, 76 to 88, and 174 to 216 MHz) and an ultra high frequency (UHF) band (470 to 698 MHz) allocated for TV broadcast, and refers to frequency bands allowed for use of unlicensed devices on condition that they do not disturb communication of licensed devices (TV broadcast, wireless microphones, etc.) operating in the frequency bands.
512 to 608 MHz and 614 to 698 MHz bands are allowed for operation of all unlicensed devices except for some special cases, but 54 to 60 MHz, 76 to 88 MHz, 174 to 216 MHz, and 470 to 512 MHz bands are allowed for communication between fixed devices only. The fixed device refers to a device which transmits data at a fixed location only. In the following description, white space bands include the above-described TVWS, but are not limited thereto.
An unlicensed device which desires to use a white space band should provide a function of protecting licensed devices. Accordingly, the unlicensed device should check whether a white space band is occupied by a licensed device, before starting transmission in the white space band. That is, the unlicensed device may be allowed to use the white space band only when a licensed device is not using the white space band.
In this regard, the unlicensed device should access a geo-location database (GDB) via the Internet or a dedicated network to acquire information about a list of channels (i.e., a set of channels) available in a corresponding area. The GDB is a database for storing and managing device information of licensed devices registered therein and channel usage information dynamically changed depending on geographical locations and channel using times of the licensed devices. To solve a problem of coexistence of unlicensed devices using the white space, a signaling protocol, e.g., common beacon frame, and a spectrum sensing mechanism may be used.
In the IEEE 802.11 system, a TVWS device may refer to an unlicensed device operating in a TVWS spectrum using a medium access control (MAC) layer and a physical (PHY) layer of IEEE 802.11. In this specification, unless otherwise mentioned, a station (STA) refers to a TVWS device operating in a TVWS spectrum.
An STA should provide a function of protecting incumbent or primary users having access priority and including licensed users (e.g., TV user, wireless microphone, etc.). That is, when an incumbent user is using TVWS, the STA should stop using a corresponding channel. Accordingly, the STA should find out a channel available to unlicensed devices (i.e., a channel not used by a licensed device) and operate on the available channel.
The STA may find out an available channel by performing a spectrum sensing mechanism or by accessing a GDB to find out a TV channel schedule. The spectrum sensing mechanism may include energy detection (a scheme for determining that an incumbent user is using a channel if the intensity of a received signal is equal to or higher than a certain value), feature detection (a scheme for determining that an incumbent user is using a channel if a digital TV preamble is detected), etc. Then, the STA should access the GDB to acquire GDB information based on location information thereof to check whether a licensed device is using a channel at the location. Access to and information acquisition from the GDB should be performed a sufficient number of times to protect the licensed device.
Upon determining that an incumbent user is using a channel immediately next to a currently used channel through the spectrum sensing mechanism or GDB, a terminal (or STA) and a base station (or access point (AP)) may protect the incumbent user by reducing transmit power.
For details of operation in TVWS, reference can be made to, for example, “Second Memorandum Opinion and Order” (FCC, ET Docket No. 10-174, September 2010.), “IEEE 802.22: The first cognitive radio wireless regional area network standard,” (C. R. Stevenson, G. Chouinard, Z. Lei, W. Hu, S. J. Shellhammer, and W. Caldwell, IEEE Commun Mag., pp. 130-138, January 2009.), “TVWS PAR and 5C,” (IEEE 802.11-09/0934r05, September 2009.), and “Enhancing Channel Availability Query to support database query for multiple locations based on multiple locations, IEEE 802.11af-11/462r4” (Y-D. Alemeseged, Y. Seok, P. Kafle, S. Shelhammer and H. Harada, April 2011).